This invention relates to hold circuits for establishing and releasing a hold condition on a telephone line and, more particularly, to improved modular hold circuits which may be independently connected to a telephone line, accessed and released from any telephone instrument connected to that telephone line and operable to maintain an enabled hold condition despite the application to that telephone line of a "call waiting signal".
In my U.S. Pat. No. 4,243,844, as issued on Jan. 6, 1981, and reissued as Re. 31,643 dated Aug. 1984 there are disclosed hold circuits for telephone systems which are highly convenient in that the same are operable with either rotary or DTMF telephone systems and are connectable to a telephone line through the same form of jack employed for conventional telephone instruments. The hold circuits there disclosed are operable within a telephone system having one or more telephone instruments connected thereto and only a single hold circuit module need be employed per telephone line. The hold circuit module may be selectively enabled and released from any telephone instrument connected to the line without, in preferred embodiments, any direct association to a given telephone instrument other than that achieved by connection to the common telephone line.
Thus, rather than requiring the separate provision of a hold button or the like at each telephone instrument, preferred embodiments of the hold circuit there disclosed may be selectively enabled from any telephone instrument connected to the line by the depression of a designated button at a DTMF pad at a telephone instrument connected to the line and subsequently released merely by lifting the handset at any telephone instrument connected to the line. In addition, the hold circuit may be engaged when the handset is in an off-hook condition without disabling the communicating function of the handset and the hold condition is retained when the handset is returned to the telephone instrument to establish an on-hook condition. The hold condition is removed upon subsequent removal of the handset from the telephone instrument to establish an off-hook condition, and the accessing and release of the hold circuit may occur at different telephone instruments connected to the same telephone line.
The hold circuits described are thus highly line of a telephone system merely through the connection of a hold circuit module to the line. Furthermore, complex installation is avoided while use of the hold circuit is quite straightforward and user friendly.
The attributes of ease of installation, as well as the creation of a friendly interface between the user and the instrument, are especially important due to the manifest changes which are occurring in the nature of telephone systems presently being created in homes and small business locations. More particularly, the wide proliferation of systems where the user purchases telephone instruments and related on-site equipment from suppliers other than the telephone company, has created a discernible trend to a market where the user, in effect, creates his own system and merely attaches the same to jacks provided by the telephone company. This trend thus mandates on-site telephone equipment which avoids the separate wiring and other installation difficulties once conveniently handled by installation technicians dispatched by the telephone company, as well as user manuals similarly provided. Furthermore, it is important that such devices be compatible with other devices already installed by a user who might not have envisioned a need for a hold circuit capability when the initial telephone instruments for the system were purchased.
While the hold circuits described in U.S. Pat. Re. 31,643 provide a highly effective solution to these needs, the recent introduction of a new form of service by the telephone company has somewhat reduced the total flexibility of the circuits employed and, in effect, have imposed a system limitation in that the same are not fully operable where a form of service known as "call waiting" is present. More particularly, the hold circuits disclosed in U.S. Pat. No. 4,243,844 rely upon a detection of a predetermined signal to enable the hold condition and a shift in voltage on the telephone line between an on-hook and off-hook condition to cause a release of a hold condition previously established.
Thus, for instance, when a telephone instrument is in an on-hook condition, the voltage between tip and ring on the telephone line will ideally be approximately 50 volts, although marked variations due to conditions on the line and distances from the central office are encountered. Conversely, when a telephone handset is in an off-hook condition, the voltage on the telephone line will typically assume a state of from 5 to 12 volts, and it is this marked change in voltage which is relied upon to cause a release of the hold condition previously established. However, in a system which provides call waiting, a click having a duration of approximately 200 milliseconds is provided at the handset to indicate to a user that an incoming call is present. This click, if supplied on a telephone line on which a hold condition has been imposed, will tend to drive the voltage level on the line to 0 volts so that the same may be interpreted by the hold circuit as an off-hook condition at a telephone instrument employed to release the line. Hence, under these circumstances, the line being held would be released and since all the telephone instruments for that line are in an on-hook condition, the call being held would be lost.
In addition, in preferred embodiments of the invention disclosed in U.S. Pat. No. 4,243,844, phase-locked loops were employed to ascertain the to signal the hold function from a telephone instrument in an off-hook condition. While such devices as phase-locked loops are highly precise and perform the frequency detection function with a high degree of accuracy, such devices require a separate power supply. This adds installation complexity which, while insubstantial, are preferably avoided.
Therefore, it is an object of the present invention to provide improved hold circuits for telephone systems.
A further object of this invention is to provide hold circuits for establishing and releasing a hold condition on a telephone line operable to maintain an enabled hold condition despite an application to that telephone line of a transitory signal condition.
An additional advantage realized by virtue of the circuit design of the instant invention is the elimination of false release of the Hold condition due to a condition known as "contact bounce" Contact bounce is a phenomenon that may occur when one restores the telephone handset onto the telephone cradle effecting the telephone hook-switch or cradle switch contacts closure. If the handset restoration is done somewhat abruptly, the contacts tend to bounce, that is they tend to close and open several times successively before settling down to a permanent closure mode. This process of contact bounce, will apply a transitory release signal condition to the telephone line, causing the release of a Hold condition in Typical Hold circuitry.
Accordingly, it is a further object of the instant invention to eliminate false Hold condition release caused by contact bounce of the telephone Hook-switch contacts and to provide Hold Release circuitry which is immune to any transitory release signals applied to the telephone line.
An additional object of this invention is to provide hold circuits which may be powered solely by voltage levels present on the telephone line.
Another object of this invention is to provide hold circuits operable to maintain a hold condition during and subsequent to the application to the telephone line of a "call waiting signal".